PWE3 Working Group Luca Martini (Ed.)
Internet Draft Cisco Systems Inc.
Expires: May 2008 Matthew Bocci (Ed.)
Alcatel-Lucent
Florin Balus (Ed.)
Alcatel-Lucent
November 2007
Dynamic Placement of Multi Segment Pseudo Wiresdraft-ietf-pwe3-dynamic-ms-pw-06.txt
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Abstract
There is a requirement for service providers to be able to extend the
reach of pseudo wires (PW) across multiple Packet Switched Network
domains. A Multi-Segment PW is defined as a set of two or more
contiguous PW segments that behave and function as a single point-
to-point PW. This document describes extensions to the PW control
protocol to dynamically place the segments of the multi segment
pseudo wire among a set of Provider Edge (PE) routers.
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Internet Draft draft-ietf-pwe3-dynamic-ms-pw-06.txt November 20071. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
2. Major Co-authors
The editors gratefully acknowledge the following additional co-
authors: Mustapha Aissaoui, Nabil Bitar, Mike Loomis, David McDysan,
Chris Metz, Andy Malis, Jason Rusmeisel, Himanshu Shah, Jeff
Sugimoto.
3. Acknowledgements
The editors also gratefully acknowledge the input of the following
people: Mike Ducket, Paul Doolan, Prayson Pate, Ping Pan, Vasile
Radoaca, Yeongil Seo, Yetik Serbest, Yuichiro Wada.
4. Introduction4.1. Scope
[MS-REQ] describes the service provider requirements for extending
the reach of pseudo-wires across multiple PSN domains. This is
achieved using a Multi-segment Pseudo-Wire (MS-PW). A MS-PW is
defined as a set of two or more contiguous PW segments that behave
and function as a single point-to-point PW. This architecture is
described in [MS-ARCH].
The procedures for establishing PWs that extend across a single PWE3
domain are described in [RFC4447], while procedures for setting up
PWs across multiple domains, or control planes are described in [PW-
SEG].
The purpose of this draft is to specify extensions to the PWE3
control protocol [RFC4447], and [PW-SEG] procedures, to enable
multi-segment PWs to be automatically placed. The proposed procedures
follow the guidelines defined in [RFC3036bis] and enable the reuse of
existing TLVs, and procedures defined for SS-PWs in [RFC4447].
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Internet Draft draft-ietf-pwe3-dynamic-ms-pw-06.txt November 2007
Native |<-------------Pseudo Wire----------->| Native
Service | | Service
(AC) | |<-PSN1-->| |<-PSN2-->| | (AC)
| V V V V V V |
| +-----+ +-----+ +-----+
+----+ | |T-PE1|=========|S-PE1|=========|T-PE2| | +----+
| |-------|.....PW.Seg't1........PW Seg't3......|----------| |
| CE1| | | | | | | | | |CE2 |
| |-------|.....PW.Seg't2.......|PW Seg't4......|----------| |
+----+ | | |=========| |=========| | | +----+
^ +-----+ +-----+ +-----+ ^
| Provider Edge 1 ^ Provider Edge 2 |
| | |
| | |
| PW switching point |
| |
|<---------------- Emulated Service -------------------->|
Figure 1: PW switching Reference Model
Figure 1 shows the architecture for a simple multi-segment case. T-
PE1 and T-PE2 provide PWE3 to CE1 and CE2. These PEs reside in
different PSNs. A PSN tunnel extends from T-PE1 to S-PE1 across PSN1,
and a second PSN tunnel extends from S-PE1 to T-PE2 across PSN2. PWs
are used to connect the attachment circuits (ACs) attached to T-PE1
to the corresponding AC attached to T-PE2. A PW on the tunnel across
PSN1 is connected to a PW in the tunnel across PSN2 at S-PE1 to
complete the multi-segment PW (MS-PW) between T-PE1 and T-PE2. S-PE1
is therefore the PW switching point and will be referred to as the
switching provider edge (S-PE). PW Segment 1 and PW Segment 3 are
segments of the same MS-PW while PW Segment 2 and PW Segment 4 are
segments of another MS-PW. PW segments of the same MS-PW (e.g., PW
segment 1 and PW segment 3) MUST be of the same PW type, and PSN
tunnels (e.g., PSN1 and PSN2) can be the same or different
technology. An S-PE switches an MS-PW from one segment to another
based on the PW identifiers. ( PWid , or AII ) How the Pw PDUs are
switched at the S-PE depends on the PSN tunnel technology: in case of
an MPLS PSN to another MPLS PSN PW switching the operation is a
standard MPLS label switch operation.
Note that although Figure 1 only shows a single S-PE, a PW may
transit more one S-PE along its path. For instance, in the multi-
provider case, there can be an S-PE at the border of one provider
domain and another S-PE at the border of the other provider domain.
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Internet Draft draft-ietf-pwe3-dynamic-ms-pw-06.txt November 20075. Applicability
In this document we describe the case where the PSNs carrying the
SS-PW are only MPLS PSNs using the generalized FEC 129. Interactions
with an IP PSN using L2TPv3 as described in [PW-SEG] section 7.4 are
left for further study.
5.1. Requirements Addressed
Specifically the following requirements are addressed [MS-REQ]:
- Dynamic End-to-end Signaling
- Scalability and Inter-domain Signaling and Routing
- Minimal number of provisioning touches (provisioning only at the
T-PEs)
- Same set of T-PEs/S-PEs for both directions of a MS-PWs
- QoS Signaling, Call Admission Control
- Resiliency
- End-to-end negotiation of OAM Capability
5.2. Changes to Existing PW Signaling
The procedures described in this document make use of existing LDP
TLVs and related PW signaling procedures described in [RFC4447] and
[PW-SEG]. Only an optional Bandwidth TLV is added to address the QoS
Signaling requirements (see "MS-PW Next Hop Bandwidth Signaling"
section for details).
6. PW layer 2 addressing
Single segment pseudo wires on an MPLS PSN use Attachment circuit
identifiers for a PW using FEC 129. In the case of an automatically
placed MS-PW, there is a requirement to have individual global
addresses assigned to PW attachment circuits, for reachability , and
manageability of the PW. Referencing figure 1 above, individual
globally unique addresses MUST be allocated to all the ACs , and S-
PEs composing an MS-PW.
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Internet Draft draft-ietf-pwe3-dynamic-ms-pw-06.txt November 20076.1. Attachment Circuit Addressing
The attachment circuit addressing is derived from [AII] AII type 2
shown here:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type=02 | Length | Global ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Global ID (contd.) | Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (contd.) | AC ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AC ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Implementations of the following procedure MUST interpret the AII
type to determine the meaning of the address format of the AII,
irrespective of the number of segments in the MS-PW.
A unique combination Global ID, Prefix, and AC ID parts of the AII
type 2 will be assigned to each AC. In general the same global ID and
prefix will be assigned for all ACs belonging to the same T-PE,
however this is not a strict requirement. A particular T-PE might
have more than one prefix assigned to it, and likewise a fully
qualified AII with the same Global ID/Prefix but different AC IDs
might belong to different T-PEs.
For the purpose of MS-PW the AII MUST be globally unique across all
interconnected PW domains.
6.2. S-PE addressing
The T-PE may elect to select a known specific path along a set of S-
PEs for a specific PW. This requires that each S-PE be uniquely
addressable in terms of pseudo wires. For this purpose at least one
AI address of the format similar to AII type 2 [AII] composed of the
Global ID, and Prefix part only MUST be assigned to each S-PE.
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Internet Draft draft-ietf-pwe3-dynamic-ms-pw-06.txt November 20077. Dynamic placement of MS-PWs
[PW-SEG] describes a procedure for connecting multiple pseudo wires
together. This procedure requires each S-PE to be manually configured
with the information required to terminate and initiate the SS-PW
part of the MS-PW. The procedures in the following sections describe
an method to extend [PW-SEG] by allowing the automatic selection of
pre-defined S-PEs, and automatically setting up a MS-PW between two
T-PEs.
7.1. Pseudo wire routing procedures
The AII type 2 described above contains a Global ID, Prefix, and AC
ID. The TAII is used by S-PEs to determine the next SS-PW destination
for LDP signaling.
Once an S-PE receives a MS-PW label mapping message containing a TAII
with an AII that is not locally present, the S-PE performs a lookup
in a local Layer 2 AII PW routing table. If this lookup results in an
IP address of the next PE that advertised reachability information
for the AII in question, then the S-PE will initiate the necessary
LDP messaging procedure for setting up the next PW segment. If the
AII PW routing table lookup does not result in a IP address of the
next PE, the destination AII has become unreachable, and the PW MUST
fail to setup. In this case the next PW segment is considered
unprovisioned, and a label release MUST be returned to the T-PE with
a status message of "AII Unreachable".
If the TAI of a MS-PW label mapping message, received by a PE,
contains the prefix of a locally provisioned prefix on that PE, but
an AC ID that is not provisioned, then the LDP liberal label
retention procedures apply, and the label mapping message is
retained.
To allow for dynamic end-to-end signaling of MS-PWs, information must
be present in S-PEs to support the determination of the next PW
signaling hop. Such information can be provisioned (static route
equivalent) on each S-PE system or disseminated via regular routing
protocols (e.g. BGP).
7.1.1. AII PW routing table Lookup aggregation rules
All PEs capable of dynamic multi segment pseudowire path selection,
must build a PW routing table to be used for PW next hop selection.
The PW addressing scheme (AII type 2 in [AII]) consists of a Global
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Internet Draft draft-ietf-pwe3-dynamic-ms-pw-06.txt November 2007
Id, a 32 bit prefix and a 32 bit Attachment Circuit ID.
An aggregation scheme similar with the one used for classless IPv4
addresses can be employed. An (8 bits) length mask is specified as a
number ranging from 0 to 96 that indicates which Least Significant
Bits (LSB) are ignored in the address field when performing the PW
address matching algorithm.
0 31 32 63 64 95 (bits)
+-----------+--------+--------+
| Global ID | Prefix | AC ID |
+-----------+--------+--------+
During the signaling phase, the content of the (fully qualified) TAII
type 2 field from the FEC129 TLV is compared against routes from the
PW Routing table. Similar with the IPv4 case, the route with the
longest match is selected, determining the next signaling hop and
implicitly the next PW Segment to be signaled.
7.1.2. PW Static Route
For the purpose of determining the next signaling hop for a segment
of the pseudo wire, the PEs MAY be provisioned with fixed route
entries in the PW next hop routing table. The static PW entries will
follow all the addressing rules and aggregation rules described in
the previous sections. The most common use of PW static provisioned
routes is this example of the "default" route entry as follows:
Global ID = 0 Prefix = 0 AC ID = 0 , Prefix Length = 0 Next Signaling
Hop = S-PE1
7.1.3. Dynamic advertisement with BGP
Any suitable routing protocol capable of carrying external routing
information may be used to propagate MS-PW path information among S-
PE, and T-PE. However, T-PE, and S-PEs, MAY choose to use Boundary
Gateway Protocol (BGP) [RFC2858] to propagate PW address information
throughout the PSN.
Contrary to other l2vpn signaling methods that use BGP [L2-
SIGNALING], in the case of the dynamically placed MS-PW if the source
T-PE knows a priori (by provisioning) the address of the terminating
T-PE. Hence there is no need to advertise a "fully qualified" 96 bit
address on a per PW Attachment Circuit basis. Only the T-PE Global
ID, Prefix, and prefix length needs to be advertised as part of well
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Internet Draft draft-ietf-pwe3-dynamic-ms-pw-06.txt November 2007
known BGP procedures - see [RFC2858].
As PW Endpoints are provisioned in the T-PEs. The ST-PE will use this
information to obtain the first S-PE hop (i.e., first BGP next hop)
to where the first PW segment will be established. Any subsequent S-
PEs will use the same information (i.e. the next BGP next-hop(s)) to
obtain the next-signaling-hop(s) on the path to the TT-PE.
The PW dynamic path NLRI is advertised in BGP UPDATE messages using
the MP_REACH_NLRI and MP_UNREACH_NLRI attributes [RFC2858]. The [AFI,
SAFI] value pair used to identify this NLRI is (AFI=25, SAFI=6
(pending IANA allocation)).
The Next Hop field of MP_REACH_NLRI attribute shall be interpreted as
an IPv4 address, whenever the length of the NextHop address is 4
octets, and as a IPv6 address, whenever the length of the NextHop
address is 16 octets.
The NLRI field in the MP_REACH_NLRI and MP_UNREACH_NLRI is a prefix
of 0 to 96 bits encoded as defined in section 4 of [RFC2858].
This prefix is structured as follows:
0 31 32 63 64 95 (bits)
+-----------+--------+--------+
| Global ID | Prefix | AC ID |
+-----------+--------+--------+
Except for the default PW route, which is encoded as a 0 length
prefix, the minimum prefix length is 32 bits. Prefix lengths of 65 to
95 are invalid as the AC ID field cannot be aggregated.
7.2. LDP Signaling
The LDP signaling procedures are described in [RFC4447] and expanded
in [PW-SEG]. No new LDP Signaling components are required for setting
up a dynamically placed MS-PW. However some optional signaling
extensions are described below.
7.2.1. MS-PW Bandwidth Signaling
In the SS-PW case the PW QoS requirements may easily be met by
selecting a MPLS PSN tunnel at the S-PE that meets the PW QoS
requirements. However in the case of an automatically placed MS-PW
the QoS requirements for a SS-PW not initiating on a T-PE MAY need to
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Internet Draft draft-ietf-pwe3-dynamic-ms-pw-06.txt November 2007
be indicated along with the MS-PW addressing. This is accomplished by
including an OPTIONAL PW Bandwidth TLV. The PW Bandwidth TLV is
specified as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| PW BW TLV (0x096E) | TLV Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forward SENDER_TSPEC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse SENDER_TSPEC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The complete definitions of the content of the SENDER_TSPEC objects
are found in [TSPEC] section 3.1. The forward SENDER_TSPEC refers to
the data path in the direction of ST-PE to TT-PE. The reverse
SENDER_TSPEC refers to the data path in the direction TT-PE to ST-PE.
In the forward direction, after a next hop selection is determined, a
T/S-PE SHOULD reference the forward SENDER_TSPEC object to determine
an appropriate PSN tunnel towards the next signaling hop. If such a
tunnel exists, the MS-PW signaling procedures are invoked with the
inclusion of the PW Bandwidth TLV. When the PE searches for a PSN
tunnel, any tunnel which points to a next hop equivalent to the next
hop selected will be included in the search.(The LDP address TLV is
used to determine the next hop equivalence)
When an S/T-PE receives a PW Bandwidth TLV, once the PW next hop is
selected, the S/T-PE MUST request the appropriate resources from the
PSN. The resources described in the reverse SENDER_TSPEC are
allocated from the PSN toward the originator of the message or
previous hop. When resources are allocated from the PSN for a
specific PW, then the PSN SHOULD account for the PW usage of the
resources.
In the case where PSN resources towards the previous hop are not
available the following procedure MUST be followed:
-i. The PSN MAY allocate more QoS resources, e.g. Bandwidth, to
the PSN tunnel.
-ii. The S-PE MAY attempt to setup another PSN tunnel to
accommodate the new PW QoS requirements.
-iii. If the S-PE cannot get enough resources to setup the segment
in the MS-PW a label release MUST be returned to the
previous hop with a status message of "Bandwidth resources
unavailable"
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Internet Draft draft-ietf-pwe3-dynamic-ms-pw-06.txt November 2007
In the latter case, the T-PE receiving the status message MUST also
withdraw the corresponding PW label mapping for the opposite
direction if it has already been successfully setup.
If an ST-PE receives a label mapping message the following procedure
MUST be followed:
If the ST-PE has already sent a label mapping message for this PW
then the ST-PE must check that this label mapping message originated
from the same LDP peer to which the corresponding label mapping
message for this particular PW was sent. If it is the same peer, the
the PW is established. If it is a different peer, then ST-PE MUST
send a label release message, with a status code of "Duplicate AII"
to the PE that originate the LDP label mapping message.
If the PE has not yet sent a label mapping message for this
particular PW , then it MUST send the label mapping message to this
same LDP peer, regardless of what the PW TAII routing lookup result
is.
7.2.2. Active/Passive T-PE Election Procedure
When a MS-PW is signaled, Each T-PE might independently start
signaling the MS-PW, this could result in a different path selected
for each T-PE PW. To avoid this situation one of the T-PE MUST start
the PW signaling (active role), while the other waits to receive the
LDP label mapping before sending the respective PW LDP label mapping
message. (passive role). The Active T-PE (the ST-PE) and the passive
T-PE (the TT-PE) MUST be identified before signaling is initiated for
a given MS-PW.
The determination of which T-PE assume the active role SHOULD be done
as follows: the SAII and TAII are compared as unsigned integers, if
the SAII is bigger then the T-PE assumes the active role.
The selection process to determine which T-PE assumes the active role
MAY be superseded by manual provisioning.
7.2.3. Detailed Signaling Procedures
On receiving a label mapping message, the S-PE MUST inspect the FEC
TLV. If the receiving node has no local AII matching the TAII for
that label mapping then the S-PE will check if the FEC is already
installed for the forward direction:
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Internet Draft draft-ietf-pwe3-dynamic-ms-pw-06.txt November 2007
- If it is already installed, and the received mapping was received
from the same LDP peer where the forward LDP label mapping was
sent, then this label mapping represents signaling in the reverse
direction for this MS-PW segment.
- Otherwise this represents signaling in the forward direction.
For the forward direction:
-i. Determine the next hop S-PE or T-PE according to the
procedures above.
-ii. Check that a PSN tunnel exists to the next hop S-PE or T-PE.
If no tunnel exists to the next hop S-PE or T-PE the S-PE
MAY attempt to setup a PSN tunnel.
-iii. Check that a PSN tunnel exists to the previous hop. If no
tunnel exists to the previous hop S-PE or T-PE the S-PE MAY
attempt to setup a PSN tunnel.
-iv. If the S-PE cannot get enough PSN resources to setup the
segment to the next or previous S-PE or T-PE, a label
release MUST be returned to the T-PE with a status message
of "Resources Unavailable".
-v. If the label mapping message contains a Bandwidth TLV,
allocate the required resources on the PSN tunnels in the
forward and reverse directions according to the procedures
above.
-vi. Allocate a new PW label for the forward direction.
-vii. Install the FEC for the forward direction.
-viii. Send the label mapping message with the new forward label
and the FEC to the next hop S-PE/T-PE.
For the reverse direction:
-i. Install the received FEC for the reverse direction.
-ii. Determine the next signaling hop by referencing the LDP
sessions used to setup the LSP in the Forward direction.
-iii. Allocate a new PW label for the reverse direction.
-iv. Install the FEC for the reverse direction.
-v. Send the label mapping message with a new label and the FEC
to the next hop S-PE/ST-PE.
7.2.4. Support for Explicit PW Path
The Explicit Route TLV format defined in [RFC3212] section 4.1 MAY be
used to signal an explicit path for a MS-PW. An Explicit PW path may
be required to provide a simple solution for 1:1 protection with
diverse primary and backup path or to enable controlled signaling
(strict or loose) for special PWs. Details of its usage to be
provided in a future study.
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Internet Draft draft-ietf-pwe3-dynamic-ms-pw-06.txt November 20078. Failure Handling Procedures8.1. PSN Failures
Failures of the PSN tunnel MUST be handled by PSN mechanisms. If the
PSN is unable to re-establish the PSN tunnel, then the S-PE SHOULD
follow the procedures defined in Section 8 of [PW-SEG].
8.2. S-PE Reachability Failures
For defects in an S-PE, the procedures defined in [PW-SEG] SHOULD be
followed. However in general an established MS-PW will not be
affected by changes in L2 PW reachability information.
T-PEs that receive a label release message with a status of "AII
Unreachable" MUST re-attempt to establish the PW immediately. However
the T-PE MUST throttle its PW setup message retry attempts with an
exponential backoff in situations where PW setup messages are being
constantly released. It is also recommended that a T-PE detecting
such a situation take action to notify an operator.
If there is a change in the L2 PW reachability information in the
forward direction only, the T-PE MAY elect to tear down the MS-PW by
sending a label withdraw message and re-establish the MS-PW. In the
same case, an S-PE MAY do the same by sending a label withdraw
message in the forward direction, and a label release message in the
opposite direction along the MS-PW.
A change in L2 reachability information in the reverse direction has
no effect on an MS-PW.
9. Operations and Maintenance (OAM)
The OAM procedures defined in [PW-SEG] may be used also for MS-PWs. A
PW switching point TLV is used [PW-SEG] to record the switching
points that the PW traverses.
In the case of a MS-PW where the PW Endpoints are identified though
using a globally unique, FEC 129-based AII addresses, there is no
PWID defined on a per segment basis. Each individual PW segment is
identified by the address of adjacent S-PE(s) in conjunction with the
SAI and TAI. In this case, the following type MUST be used in place
of type 0x01 in the PW switching point TLV:
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Internet Draft draft-ietf-pwe3-dynamic-ms-pw-06.txt November 2007
Type Length Description
0x06 8 L2 PW address of PW Switching Point
The above field MUST be included together with type 0x02 in the TLV
once per individual PW Switching Point following the same rules and
procedures as described in [PW-SEG].
10. Security Considerations
This document specifies only extensions to the protocols already
defined in [RFC4447], and [PW-SEG]. Each such protocol may have its
own set of security issues, but those issues are not affected by the
extensions specified herein. Note that the protocols for dynamically
distributing PW Layer 2 reachability information may have their own
security issues, however those protocols specifications are outside
the scope of this document.
11. IANA Considerations
This document uses several new LDP TLV types, IANA already maintains
a registry of name "TLV TYPE NAME SPACE" defined by RFC3036. The
following value is suggested for assignment:
TLV type Description
0x096E Bandwidth TLV
11.1. LDP Status Codes
This document uses several new LDP status codes, IANA already
maintains a registry of name "STATUS CODE NAME SPACE" defined by
RFC3036. The following values have been pre-allocated:
Range/Value E Description Reference
------------- ----- ---------------------- ---------
0x00000037 0 Bandwidth resources unavailable RFCxxxx
0x00000038 0 Resources Unavailable RFCxxxx
0x00000039 0 AII Unreachable RFCxxxx
0x0000003A 0 PW Loop Detected RFCxxxx
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Internet Draft draft-ietf-pwe3-dynamic-ms-pw-06.txt November 200711.2. BGP SAFI
IANA needs to allocate a new BGP SAFI for "Network Layer Reachability
Information used for Dynamic Placement of Multi-Segment Pseudiwires"
from the IANA "Subsequence Address Family Identifiers (SAFI)"
registry. The following value has been pre-allocated:
Value Description Reference
----- ----------- ---------
6 Network Layer Reachability Information used [RFCxxxx]
for Dynamic Placement of Multi-Segment
Pseudowires
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